The long-term goals of the work proposed in this grant application are to study neural plasticity of the adult mammalian visual system by adding new sensory input. Experiments that enhance rather than ablate sensory input offer a new avenue of research for answering fundamental questions about the plasticity of the adult visual system and have important implications for the rational design of gene therapy and retinal prostheses. We have chosen red-green color vision as a model system in which gain of function can be monitored quantitatively at the level of the retina using the electroretinogram and at the level of the visual cortex with behavioral tests of color vision. As a first step toward achieving our long term goals, we propose two specific aims. Specific Aim 1. To determine whether adding a third cone type via viral mediated delivery of a photopigment gene to the retina in a dichromatic adult squirrel monkey can expand his sensory capacity and change color vision behavior from dichromatic to trichromatic. Specific Aim 2. To determine whether expressing a long-wavelength sensitive pigment gene in a subset of UV cones via viral mediated gene delivery to the gerbil retina can expand the gerbil's color vision capacity to include making color discriminations in the red-green region of the spectrum.